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German tourist Jurgen Kohl was window shopping in Mexico's trendy Zona Rosa district last Friday afternoon when he saw a flash of brilliant light followed by a thundering bang and the sound of shattering glass. Minutes later, ambulances and police rushed into the street to find three blood-soaked people crying for help and the remains of a crude plastic explosives device. One of the injured died on the way to hospital. "Being in Mexico, I first thought it was an earthquake or something," Kohl said, visibly shaken from the blast. "I had no idea that bombs went off here."

No group has claimed responsibility for the bombing, which sent shock waves through the heart of the Mexican capital. But officials on the case have said they are investigating whether it could be the work of drug cartels reeling from a crackdown by President Felipe Calderon. Leftist guerrilla groups also have a history of bombing in Mexico, but they have normally hit political or commercial buildings at night without leaving casualties.

If drug gangs are proven to be behind the blast, it would show a worrying escalation in the battle between organized crime and the government. Since Jan. 1, drug cartel gunmen armed with huge arsenals of automatic rifles and grenade launchers have slain more than 30 police, soldiers and judges in ambushes and assassinations. The attacks come as Calderon has made record drug busts, sent 25,000 police and soldiers against the gangs, and extradited alleged kingpins to the United States.

While the Mexican crime families do not have a history of using bombs, explosive devices used to be a favored tactic of their associates in Colombia. In the 1980s and 1990s, the Medellin cartel responded to a government crackdown with bombs on street corners, cars and even one passenger jet, killing hundreds. Colombian gangsters have long been selling cocaine to the Mexican cartels, who smuggle it into the United States. "The cartels could be turning to this Colombian tactic of using terror to pressure the government to back off," said Mexican drug expert Jorge Chabat. "They may be trying to raise the political cost for Calderon of carrying out his campaign."

Police say the man killed in the blast was probably himself carrying the bomb because the device went off about a three feet from the ground, blowing out surrounding car windows but not causing a crater. "The man completely lost his hand. He surely did this when he was handling the explosive device," Mexico City Police Chief Joel Ortega told reporters.

If this is the case, the bomb may have gone off before it was planned, the detectives say. It exploded two blocks from Mexico City's police headquarters. A woman who received burns from the blast was being questioned by detectives in her hospital bed Monday while under police guard. Identified as Tania Vazquez, 22, the woman lives in the rough market neighborhood of Tepito, known as a center of drug dealing. Police raided her home on Sunday and took about $2,500 in cash and lists of phone numbers, officials said. No charges have been leveled against her at this time.

Living several blocks away from Vasquez was an alleged drug trafficker working for the powerful Sinaloa cartel named Rogelio Mena. Police had arrested Mena two days before the blast, along with six other men and an arsenal of weaponry including a Barrett anti-aircraft gun. He is being held for 90 days on suspicion of racketeering and weapons offenses. The Sinaloa cartel has been blamed for carrying out many of the attacks on police and soldiers this year. Federal agents say they also foiled an attempt by the gang to assassinate Federal Prosecutor Jose Luis Santiago, who oversees the extradition of high-level criminals to the United States.

The last bombings in the country were carried out by a leftist guerrilla group called the People's Revolutionary Army, which blew up sections of two pipelines of state oil monopoly Pemex in September. That group previously targeted other oil installations, banks and political buildings and has always claimed responsibility for its attacks.